Angle sensor

ABSTRACT

The invention provides an angular position sensor ( 1 ) comprising two magnets ( 32, 33 ) attached to the inside face of a rotor ( 31 ) for generating a magnetic field ( 50 ) and a magnetism detection device ( 43 ) disposed in the magnetic field ( 50 ). The magnets ( 32, 33 ) are arranged substantially opposite each other with a pole of opposite polarity of each magnet ( 32, 33 ) directed towards the centre of the rotor ( 31 ). A uniform magnetic field ( 50 ) is generated around the magnetism detection device ( 43 ) which provides more accurate sensor measurements over both small and large angles of rotation of the rotor ( 31 ). Furthermore, the magnets ( 32, 33 ) need only be axially magnetised which simplifies construction of the sensor.

The present invention relates to an angular position sensor comprisingtwo magnets attached to a rotor for generating a magnetic field and amagnetism detection device disposed in the magnetic field for detectingchanges in the magnetic field on rotation of the rotor so that theangular position of the rotor is detected based on the changes in themagnetic field to provide an output signal from the magnetism detectiondevice indicating the angular position of the rotor. In particular theinvention relates to an angular position sensor for use in theautomotive industry.

A position sensor is an electromechanical device used to convey theposition of an item or an object to a control device. In industry, theseposition sensors are known as non-contacting position sensors. Strictlyspeaking, it is not true to say non-contacting as there is contactbetween the bearing faces of so called non contacting sensors. However,the position or angle detecting components do not come into contact witheach other and as a result are more resilient to wearing out, hence theterm non-contacting. The invention is primarily concerned withnon-contacting sensors.

In industry it is highly desirable to provide a low cost, easy tomanufacture and accurate angular position sensors. In the automotiveindustry it is envisaged that angular position sensors will be installedin many if not all future accelerator pedal applications due toenvironmental legislation requiring greater control of the automotivecombustion process. The application requires accurate sensing over smallangles of rotation. Angular position sensors are also required forthrottle valves and exhaust gas recirculation valves which requireaccurate sensing over large angles of rotation. As automotivemanufacturers usually utilize the same technology for similar butdifferent applications throughout a vehicle it is necessary to be ableto mass produce an angular position sensor which is accurate fordetecting both small and large angles of rotation.

The angular position sensors generally disclosed by the prior artutilise one radial magnetised magnet only in the rotor for generating amagnetic field. A major problem is that this radial magnetisationrequirement means that the magnet is difficult and hence more expensiveto manufacture. A further problem is that the volume of magneticmaterial necessary to make the magnets makes these position sensors moreexpensive. Examples of prior art angular position sensors are disclosedin U.S. Pat. No. 5,789,917, European Publication Nos. EP 0514530 and EP0665416 (all Moving Magnet Technology Inc.)

A position sensor is disclosed by U.S. Pat. No. 5,818,223 (DurakoolInc.) which discloses an angular position sensor for sensing the angularposition of a pivotally mounted device attached to a rotor disposed in apredetermined housing about a rotor axis. Again, this US PatentSpecification discloses one magnet only, which is circular in shape.This leads to the problem that the tooling required to manufacture isexpensive while magnetising the magnets is difficult because the magnetsneed to be magnetised radially to achieve high accuracy.

U.S. Pat. No. 6,472,865 (Wabash Technologies Inc.) discloses a magneticrotational position sensor having one magnet only connected to theinside face of a rotor in which one of the poles of the magnet isdirected towards the centre of the rotor. However the magnetic fieldprematurely flows to the rotor due to the lack of stator pieces to guideit and hence does not flow at 90 degrees to the magnetism detectiondevice, hence the sensor is not accurate especially for large angularchanges of rotation of the rotor.

A major problem with existing non-contacting angular position sensors asdescribed above is that they are not accurate for both small angles andlarge angles of rotation. European Publication No. EP 1143220 (DensoCorporation) discloses an angular position detection device which has arotor which comprises two magnets. The two magnets are partiallysemi-circular in shape and the poles of the magnet are arranged aroundthe circumference of the rotor. A problem with this European patentspecification is that this technology relies on offsetting the statorpieces and magnetism detection device from the axis of rotation of therotor and magnets. It also relies on two circumferentially magnetisedmagnets which are semicircular in cross section. This means that whilethe sensor is relatively accurate, it is not accurate for sensing bothsmall angles and large angles of rotation. This is a particular problem,for example, in the automotive industry which requires angular positionsensors which are accurate over a small and large range of rotation forexample foot pedals of a vehicle and engine throttle valve sensing ofthe vehicle. This European publication further discloses a singularmagnet in which one of the pole ends is faced towards the centre ofrotation of the rotor. However, again this suffers from the same problemas U.S. Pat. No. 6,472,865 in that the offset magnetic field will notproduce high accuracy over small and large angles of rotation. It alsosuffers partially from the same problem as U.S. Pat. No. 6,472,865 inthat although there are flux concentrators without a second magnet toensure the magnetic field is pulled through the magnetism detectiondevice there will be premature field deviation to the rotor hence onceagain affecting the accuracy.

The present invention is therefore directed towards providing anaccurate low cost angular position sensor, and in particular for use inthe automotive industry, to overcome the above-mentioned problems.

STATEMENTS OF INVENTION

According to the present invention there is provided an angular positionsensor comprising two magnets attached to a rotor for generating amagnetic field and a magnetism detection device disposed in the magneticfield for detecting changes in the magnetic field on rotation of therotor so that the angular position of the rotor is detected based on thechanges in the magnetic field to provide an output signal from themagnetism detection device indicating the angular position of the rotorcharacterised in that the magnets are positioned substantially oppositeeach other, a pole of one magnet facing a pole of the other magnet andthe two other poles facing away from each other.

The advantage of the present invention is that it provides a moreaccurate sensor over large and small angles of rotation and taking intoaccount that radial magnetization cannot be achieved fully in practicethis means that actual sensors will have a larger accuracy margin overthe prior art. The term substantially opposite means one pole faces apole of the other magnet substantially along the one diameter of therotor. Preferably the axis line formed by the two magnets substantiallyintersects the centre axis of rotation of the rotor.

Ideally the facing poles of each magnet are of opposite polarity.

In one embodiment there is provided an angular position sensor in whicha flux concentrating stator is provided comprising a pair of statorpieces positioned substantially on either side of the magnetismdetection device.

The use of two magnets and two stator pieces ensures that magnetic fluxflows through the magnetism detection device and there is no straying ofthe magnetic field prematurely i.e. before it flows through themagnetism detection device. Also that the magnetic field remains forexample at 90 degrees approx. to the face of the magnetism detectiondevice and hence the magnetism detection device only experiences avariation in inductance as the rotor is rotated. With some of the priorart sensors i.e. Wabash and the Denso single magnet there will bemagnetic field straying prematurely to the rotor and the field will notalways be at for example 90 degrees to the magnetism detection devicehence diminishing accuracy.

In another embodiment the stator pieces are substantially on the sameaxis line formed by the two magnets when in a fully saturated position.

In a further embodiment the magnetism detection device and the statorpieces are positioned offset from the centre of the rotor.

Preferably the magnets are attached to the inside face of the rotor.

Ideally the magnetism detection device is one of a hall effect device,an integrated circuit or a GMR device.

Preferably the magnets are axially magnetised.

It will be appreciated that the present invention only uses magnetswhich have to be axially magnetised. Radial and various othermagnetising requires complicated and expensive magnetising fixtures orheads which require constant calibration and long cycle times hencepushing up the component cost. Also due to the current method ofmanufacture of these fixtures or heads true radial magnetisation is notachievable leading to decreased actual accuracy. Axial magnets aresimple and less expensive to produce and hence the above concerns do notapply.

Ideally the rotor comprises a substantially circular material to form amagnetic circuit around the magnetism detection device.

Preferably the substantially circular material is iron or a magneticalloy.

In one embodiment the rotor is pivotally connected to a pedal arm formounting to a vehicle.

In a further embodiment the rotor is pivotally connected to a throttlevalve for sensing the position of the throttle valve.

Although the sensor is of similar overall size to the prior art themagnetic components and the stator components are of simpler and smallergeometry which results in less material being used and much simpler andhence less expensive tooling.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example only,with reference to the accompanying drawings, in which:—

FIG. 1 is a front perspective view of an angular position sensor,

FIG. 2 is an exploded front perspective view of the angular positionsensor of FIG. 1,

FIG. 3 is an exploded rear perspective view of the angular positionsensor of FIG. 1,

FIG. 4 is a perspective view of a rotor assembly according to thepresent invention with a stator and a magnetism detection devicepositioned in the centre of the rotor,

FIG. 5 is a part diagrammatic plan view of FIG. 4 illustrating amagnetic field,

FIGS. 6(a) to (e) are graph results of different tests carried out onthe angular position sensor, and

FIG. 7 is a perspective view of an angular position sensor pivotallyattached to a foot pedal of a vehicle.

Referring to FIGS. 1 to 3 inclusive, there is illustrated an angularposition sensor according to the present invention illustrated generallyby the reference numeral 1. The sensor 1 comprises a housing 2 connectedto a rotor assembly 3 which in turn communicates with a PCB carrierassembly 4, all of which is encased by a cover 5. The rotor assembly 3comprises a rotor 31 in which there is placed a pair of magnets 32, 33on the inside face 34 of the rotor 31. The operation of the rotorassembly 3 will be discussed in more detail below.

Referring to FIG. 4 there is illustrated part of the rotor assembly,indicated generally by the reference numeral 30. The rotor 31, which isrotatable, has two magnets 32, 33, each magnet positioned substantiallyopposite each other and positioned on the inside face 34 of the rotor 31forming a magnetic axis line. The PCB carrier assembly 4 has attached amagnetism detection device 43 which detects changes in inductance. Astator in the form of two stator pieces 41, 42 also attached to the PCBcarrier assembly 4, are positioned either side of the magnetismdetection device 43. The stator pieces 41, 42 are on an axisperpendicular to the front 44 and rear 45 faces of the magnetismdetection device 43. When the angular position sensor 1 is fullyassembled, the magnetism detection device 43 and the stator pieces 41and 42 are positioned inside the rotor 31. It will be appreciated thatthe PCB carrier assembly 4 may be designed to be preassembled with thehousing 5.

Referring to FIG. 5, the pair of axially magnetised magnets 32, 33generate an axial magnetic field 50, in which the magnetism detectiondevice 43 and stator pieces 41, 42 are positioned. It can be seen fromFIG. 5 that the pair of magnets 32, 33 generate a symmetrical axialmagnetic field 50 around the magnetism detection device 43. The poleface of each magnet 32, 33 faced toward the centre of the rotor 31 areof opposite polarity. For example the north pole N, of the magnet 32face towards the centre means that the south pole S, of the magnet 33faces the centre of the rotor 31. It will be appreciated that the polescan be positioned the other way round. The magnetic field 50 flows inthe direction as shown in FIG. 5. The area between the two magnets 32,33 forms an air gap. The stator pieces 41, 42 improves the fluxconcentration which ensures a closed loop symmetrical magnetic field 50also ensures that the magnetic field lines are at approximately 90degrees to the magnetism detection device, and also increases theaccurate linear portion of the output curve is formed around themagnetism detection device 43.

FIGS. 6(a) to 6(e) disclose test results carried out on the sensorshowing the linearity error versus the angular displacement andinductance output versus the angular displacement generated over a broadrange of angular displacement which was modelled and evaluated oncomputer aided engineering software.

FIG. 7 discloses an application of the sensor 1 according to the presentinvention which illustrates the sensor is pivotally connected to a pedalarm for mounting to a vehicle which is pivotally attached to the rotor31 (not shown).

In operation, the two magnets 32, 33 and the rotor 31 rotate around themagnetism detection device 43. The stator pieces 41, 42 and themagnetism detection device 43 are stationary and are securely attachedto one of the faces of the PCB carrier 4. The rotor 31 which issubstantially circular in shape provides a return closed loop for themagnetic field 50 which flows through the centre of the rotor 31, aswell as reducing the effects from external interference and reducinginductance loss from the sensor. FIG. 5 depicts the sensor in the fullysaturated position i.e. maximum inductance at the sensing element of themagnetism detection device. In this position the axis through the statorpieces 41, 42 is aligned with the axis line provided by the two magnets32, 33 which provides a greater flux linkage for the magnetic fieldwhich allows for the symmetrical magnetic field 50 around the magnetismdetection device 43. In practice the stator pieces 41, 42 and magnetismdetection device 43 are angled to the axis line formed by the twomagnets 32, 33 depending on the application required. The two magnets32, 33 attached to the inside of the rotor 31 have been axiallymagnetised. As the rotor 31 and magnets 32, 33 rotate in response to anexternal force, the amount of which the rotor 31 rotates induces achange in magnetic flux which is detected by the magnetism detectiondevice 43 which is then converted to an output signal indicative of theamount the rotor 31 has rotated. The magnetism detection device 43 maybe a Hall effect device or integrated circuit which detects the inducedchange in magnetic flux which is then converted to a voltageproportional to the induced change in magnetic flux. The voltage outputwhich is linear is a signal which indicates the amount the rotor 31 hasrotated. It will be appreciated that GMR (Giant Magneto Restrictive) andintegrated circuits can also be used which detect the angle of themagnetic field.

The main feature of the present invention is to provide a pair ofmagnets 32, 33 attached to the inside face 34 of the rotor 31 having apole of opposite polarity directed towards the centre of the rotor 31and positioned substantially opposite each other. It will be appreciatedthat the magnets 32, 33 need only be magnetised axially which is mucheasier to magnetise and less expensive. The prior art discloses twocircular magnets on the rotor which requires each magnet to becircumferentially magnetised. This is difficult and expensive toachieve. One prior art sensor discloses one magnet attached to theinside face of the rotor which is axially magnetised. However, this asalready discussed is not a very accurate sensor for both small and largeangular displacements of the rotor 31.

Another important feature of the present invention is the shape of thestator which is provided by the stationary stator pieces 41, 42positioned substantially at either side of the magnetism detectiondevice 43 which concentrates the flux to provide a more accuratemeasurement of the angular displacement of the sensor. The prior artdiscloses circular pieces for concentrating the flux. The stator pieces41, 42 are of simpler and smaller geometry this saves on material costs,while at the same time improves the linearity output signal from themagnetism detection device.

It is important that the sensor 1 provides a linear output signalrepresentative of the angular displacement of the rotor 31 over a broadrange of angular displacement. The magnetism detection device 43 which,as already mentioned, may be a Hall effect device or an integratedcircuit, stores software which reads the inductance value of thechanging magnetic field in response to the rotor 31 rotating andallocates a proportional voltage to the inductance observed. Anyexternal control device (not shown) for example an engine managementunit, which reads the output from the sensor, can determine the angle ofrotation of the rotor 31 which is proportional to the generated voltage.As the external control device only reads voltage, it is important tohave equal incremental angular movements resulting in equal inductanceincrements and hence, equal voltage increments, for a particular angularrotation. For this reason, the software within the control devicerequires a linear voltage output when voltage is plotted against theangular displacement of the rotor 31. This is particularly important,for example, automotive applications, as illustrated in FIG. 7. Therotor is pivotally connected to an accelerator pedal 71 of a vehicle. Asthe pedal 71 is pressed or depressed, the rotor 31 rotates. As more andmore vehicles are now controlled by electronics, it is important toprovide an electrical signal to control the vehicle in response to thepedal 71 accurately over small angles of rotation. Another feature ofthe present invention is that the rate of change of voltage measured candetermine the speed at which a foot pedal is pressed to provide anappropriate control signal to the vehicle.

FIG. 6 a is a graph of the inductance variation generated when the rotoris rotated through 40 degrees and also the linearity error of theplotted straight line output i.e. the linear line is the output plottedagainst the angle of rotation and the curved line is the associatedlinearity error of that line. The rotor 31 and stator pieces 41, 42 aresymmetric about the same axis. The inductance is measured at the centreof the rotor 31 by the magnetism detection device 43 which is convertedinto a directly proportional voltage output by the magnetism detectiondevice 43, for example a Hall device. An Engine Management unit (notshown) has this straight line output programmed into its memory andhence by monitoring the voltage output of the device the angle ofrotation of the pedal or valve for example is known. It is also possiblefor the Engine Management Unit to monitor the acceleration of the pedalor valve and hence the sensor can become an accelerator sensor bymeasuring the rate of change of voltage. The magnetism detection device43 may incorporate temperature compensation software which corrects theoutput in cases of temperature variation if required.

FIG. 6 b is the same as FIG. 6 a but for a rotation of 100 degrees.

FIG. 6 c is the same as FIG. 6 b but the magnetism detection device 43plus stator pieces 41, 42 is offset from the rotor 31 centre in x and yby 0.25 mm to take account of mechanical tolerances and to show theeffect of wear with age on the bearing faces. The three output curvesare depicting linearity error with no offset, 0.25 mm in the Y directionand 0.25 mm in the X direction. This demonstrates a large improvementover the prior art in that it is four times more accurate which isillustrated in the table below.

FIG. 6 d is the same as FIG. 6 c but the graph is of the outputinductance, proportional to voltage, plotted against the angle ofrotation of the rotor.

FIG. 6 e is the same as FIG. 6 a but this depicts the linearity error ofthe sensor over different angles of rotation ranging from 20 degrees to180 degrees. Note that the accuracy at larger angles of rotation can beenhanced by offsetting the stator pieces 41, 42 plus the magnetismdetection device 43 in the direction of 90 degrees from the axis lineformed by the two magnets 32, 33 in the fully saturated position, seeFIG. 5.

The following table compares the sensor of the present invention withother similar type sensors in the marketplace, namely, against sensorsmanufactured by Moving Magnet Technology (MMT) Corporation, WabashCorporation and CTS Corporation over a range of tests: KEANE SENSOR TYPEINVENTION MMT WABASH CTS Direction of Axial Radial Axial Axialmagnetisation Linearity <0.05%  <0.1% <0.3% Dependent on +/− 20° onparts quality Linearity  <0.1% <0.15% <2.0% Dependent on +/− 50° onparts quality Influence of off- <0.25%  <1.0% Very high Very highcentring on linearity error CONCLUSION Good Good Low Low sensitivitysensitivity sensitivity sensitivity Good Good Large air High linearityon linearity gap sensitivity short and on short Non linear to off- largeand on large centring strokes large strokes High Very low strokessensitivity sensitivity Low to to off- sensitivity temperature centringto off- centring

As can be seen, the linearity error of the output voltage of the presentinvention is much more favourable than any other sensors of the threecompanies compared.

In a preferred embodiment of the present invention, the axis line formedby the two magnets 32, 33 substantially intersects the centre axis ofrotation of the rotor 31. Further, the stator pieces 41, 42 arepositioned substantially either side of the centre axis and onsubstantially the same axis line formed by the two magnets 32, 33 atfull saturation. However, it will be appreciated that in anotherembodiment, the axis line formed by the two magnets 32, 33 may be offsetfrom the centre axis of rotation of the rotor 31 in some applications.If the axis line through the two magnets 32, 33 and another axis lineformed by the two stator pieces 41, 42 and align the two through axis ofrotation at the rotor 31 then the inductance seen by the magnetismdetection device 43 is fully saturated or at a maximum. When these twoaxis lines are at 90 degrees. Then the magnetism detection device 43experiences zero inductance. Hence the resting point of the rotor 31varies depending on the angle of rotation being monitored for examplemonitoring at 90 degrees. Then the rest point will be 45 degrees. At oneside of this axis or zero inductance position in a clockwise directionand the maximum angular rotation will be 45 degrees. In the otherdirection 45 degrees from this axis, hence counter clockwise thus giving90 degrees angular sensing.

It will also be appreciated that the stator pieces 41, 42 can be ofvarious cross-sectional shape or size. However, it has been found that aparticular suitable pole piece cross-sectional shape is a square or asemi-octagonal pole piece. The magnetism detection device 43 may have asingle, dual output or quadruple output option depending on theapplication required.

The rotor 31 which is made of iron or magnetic alloy provides protectionfrom electrical interference and ensures a closed loop path for themagnetic field 50. The rotor 31 does not have to be circular and may beeliptical or of a shape that provides a closed loop magnetic path.Further it will be appreciated that means may be provided to rotate themagnets 32, 33 only on the inside face of the rotor.

It will be appreciated that the invention will work without the statorpieces 41, 42. What the stator pieces 41, 42 do is increase the linearportion of the output inductance/voltage curve as well as increasingaccuracy i.e. less linearity error. However, if the angular rotationalrequirement application is very small, one may find that there is aportion of the curve which is of sufficiently linear accuracy of thesensor 1. The stator pieces 41, 42 are usually required for largeangular rotations. However, for sensors of less strict accuracyrequirements and small angles of rotation, it may be possible tomanufacture the sensor without the stator pieces 41, 42.

The sensor of the present invention is capable of accurately sensinglarge and small angular rotations of a device and hence is suitable forapplication to, such as intelligent valves, pedal position sensing, asillustrated in FIG. 7, throttle position sensing and exhaust gasrecirculation valve position sensing which are being used more and morein automotive applications.

It will be appreciated that while an angular position sensor isdisclosed for automotive applications the sensor may be used in otherindustrial applications.

In the specification the terms “comprise, comprises, comprised andcomprising” or any variation thereof and the terms “include, includes,included and including” or any variation thereof are considered to betotally interchangeable and they should all be afforded the widestpossible interpretation and vice versa.

The invention is not limited to the embodiment hereinbefore described,but may be varied in both construction and detail within the scope ofthe appended claims.

1. An angular position sensor (1) comprising two magnets (32, 33) attached to a rotor (31) for generating a magnetic field (50) and a magnetism detection device (43) disposed in the magnetic field (50) for detecting changes in the magnetic field (50) on rotation of the rotor (31) so that the angular position of the rotor (31) is detected based on the changes in the magnetic field (50) to provide an output signal from the magnetism detection device (43) indicating the angular position of the rotor (31) characterised in that the two magnets (32, 33) are positioned substantially opposite each other, a pole of one magnet facing a pole of the other magnet and the two other poles facing away from each other.
 2. An angular position sensor (1) as claimed in claim 1, in which the axis line formed by the two magnets (32, 33) substantially intersects the centre axis of rotation of the rotor (31).
 3. An angular position sensor (1) as claimed in claim 1 in which the facing poles of each magnet (32, 33) are of opposite polarity.
 4. An angular position sensor (1) as claimed in claim 1, in which a flux concentrating stator is provided comprising a pair of stator pieces (41, 42) positioned substantially on either side of the magnetism detection device (43).
 5. An angular position sensor (1) as claimed in claim 4 in which the stator pieces (41, 42) are substantially on the same axis line formed by the two magnets (32, 33) when in a fully saturated position.
 6. An angular position sensor (1) as claimed in claim 1 in which the magnetism detection device (43) and the stator pieces (41, 42) are positioned offset from the centre of the rotor (31).
 7. An angular position sensor (1) as claimed in claim 1, in which the magnets (32, 33) are attached to the inside face (34) of the rotor (31).
 8. An angular position sensor (1) as claimed in claim 1, in which the magnetism detection device (43) is one of a hall effect device, an integrated circuit or a GMR (Giant Magneto Restrictive) device.
 9. An angular position sensor (1) as claimed in claim 1, in which the magnets (32, 33) are axially magnetised.
 10. An angular position sensor (1) as claimed in claim 1, in which the rotor (31) comprises a substantially circular material to form a magnetic circuit around the magnetism detection device (43).
 11. An angular position sensor (1) as claimed in claim 9, in which the substantially circular material is iron or a magnetic alloy.
 12. An angular position sensor (1) as claimed in claim 1, wherein the rotor (31) is pivotally connected to a pedal arm (71) for mounting to a vehicle.
 13. An angular position sensor (1) as claimed in claim 1, wherein the rotor (31) is pivotally connected to a throttle valve for sensing the position of the throttle valve. 